Java 9 Regular Expressions

Examples using quantifiers

Let's look at few examples to understand these basic quantifiers better.

Which regex pattern should be used to match a two-digit year or a four-digit year?

    \d{2}|\d{4}

Which regex pattern should be used to match a signed decimal number? The pattern should also match a signed integer number:

    ^[+-]?\d*\.?\d+$

Here is the breakup of the preceding regex pattern:

• The ^ and $ symbols are the start/end anchors
• The [+-]? pattern makes either the + sign or the - sign (optional because of ?) at the start
• The \d* pattern matches zero or more digits
• The \.? pattern matches an optional dot (.) literally
• The \d+ pattern matches one or more digits

The preceding regex will match all of these inputs:

• .45
• 123789
• 5
• 123.45